rod and main bearings

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I am curious as to why they would sell a set of main or rod bearings if each jounal and respective surface have different sizes. I went to Honda after searching for a 'set' or rod bearings, only to find that each rod will have a different bearing size according to rod/journal size.
Are the 'sets' that you can buy supposed to be machined down to the right size? seems a little odd that I could order the exact ones I need from honda but not from anyone else.